MIMO refers to a system with multiple inputs and multiple outputs. The definition of MIMO system is extended to wireless communication topologies in which multiple modulated signals, separated in frequency or space domain, are simultaneously transmitted through a single/multiple branch radiofrequency (RF) front-end.
MIMO systems, with modulated signals separated in space domain, refer to wireless topologies with multiple branches of RF front-ends, with all branches simultaneously involved in signal transmission. These types of MIMO systems are considered as Multi-branch MIMO systems.
MIMO systems, with modulated signals separated in frequency domain, refer to systems where multiple signals modulated in different carrier frequencies are concurrently transmitted through a single branch RF front-end. These types of MIMO systems are considered as Multi-frequency MIMO systems. Examples of multi-frequency MIMO systems are concurrent dual-band and multi-carrier transmitters. The system in frequency domain comprises two independent baseband signals as the multiple inputs and two up-converted and amplified signals at two carrier frequencies as the multiple outputs. In fact, this type of MIMO system uses a single branch RF front-end to transmit multiple signals.
RF MIMO systems are composed of linear and nonlinear components and/or sub-blocks which may results in signal quality degradation. For example, the power amplifier (PA) is one of the main building blocks of the RF front-end that has a significant nonlinear behavior. This nonlinear relation between the input signal and the amplified output signal of the transmitter results in significant distortions on the output signal. These distortions significantly degrade the output signal's quality and result in poor data communications. In this regard, different techniques to compensate for these distortions were proposed in order to improve the linearity of the RF radio front-end.
Also, there are unwanted and unavoidable interactions and correlations between the different signals in a MIMO system. These interactions are combined with the linear and nonlinear distortions in each branch of the MIMO system to generate more complex distortion effects, which considerably degrade the performance of the MIMO system. The effect of these complex distortions cannot be eliminated or reduced with conventional signal processing algorithms applied to Single Input Single Output (SISO) systems.
Therefore, there is a need for a signal processing technique for MIMO systems that compensates for any distortion, interactions, and crosstalk in the system in order to improve the signal quality of the transmission link.